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Planetary News: Near Earth Objects (2008)

Red Planet Spared Asteroid Impact

By Amir Alexander
January 10, 2007
2007 WD5 moving through the sky
Click to enlarge >
2007 WD5 moving through the sky
Image of asteroid 2007 WD5 taken by the University of Hawaii 2.2 meter telescope on Mount Mauna Kea. The circled dot is the asteroid, whereas the other dots are artifacts of cosmis rays. The stars are smeared because the telescope followed the asteroid as it moved through the sky. Credit: Tholen, Bernardi, Micheli, with support from the NSF

Mars has been declared safe. Asteroid 2007WD5, which in late December appeared as if it might be headed for a collision with Mars, will almost certainly pass thousands of miles from the planet. And while this is good news for any living inhabitants of the Red Planet --if such exist -- scientists are disappointed. They have never seen an asteroid strike a rocky planet, and were hoping for a front row view of this rare astronomical event.

But not this time. According to Steve Chesley, Paul Chodas, and Don Yeomans of NASA's Near Earth Object Program Office, the chances of this space rock hitting Mars are now only 1 in 10,000.  Such odds, they said, effectively rule out a possible collision.

2007 WD5 was originally singled out as a potential Mars impactor in the middle of December, when the odds of a strike were put at 1 in 75. The odds kept climbing through late December and early January, reaching as high as 1 in 30 at one point. But new tracking data received from four different observatories has narrowed down the uncertainties regarding the asteroid's has trajectory. As a result the odds of an impact have plummeted to insignificance. According to best estimates the asteroid will be a full 26,000 kilometers from the planet on its closest approach at noon (UTC), January 31, 2008.

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This sequence in which the chances of an impact increase steadily before suddenly plunging is a familiar one to NEO observers, according to Chesley and his colleagues. Early on the trajectory of the asteroid is not well know, and the area of uncertainty containing the planet is large. As a result, while an impact is possible, the odds of one actually taking place are long. Over time additional observations narrow down the uncertainty region, but as long the planet is still included in this region, the odds of an impact will grow steadily. Eventually the uncertainty region is narrowed down to the point where it no longer contains the planet, causing the odds of an impact to crash.

This is precisely what happened in the case of 2007 WD5. Scientists anxious to observe an asteroid impact in real time will have to wait for another day.

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